Unlike the poisonous snake sometimes referred to as the "water moccasin", cotton mouth is a condition by which your mouth is severely dried out, so much so that it feels like you in fact have cotton in your mouth. Pretty straightforward, huh? The numbing sensation can kill your sense of taste, causing "food to turn to ashes in your mouth." Of course, cotton mouth is an indication of potentially more serious health risks. So, what causes cotton mouth, and how can you defeat it?
The usual cause of cotton mouth is dehydration - playing or exercising a long time outside on a hot day will often cause your saliva glands to simply dry up as your bodily fluids are concentrated elsewhere. Another common cause of cotton mouth is, as the softlinks indiciate, smoking. Marijuana acts as a particularly strong catalyst in drying out one's throat, mouth, and lips. Other causes include sugary liquids (Kool-Aid is not good for summer time thirst!), certain medications (check the packaging - remember Jim Carrey in Me, Myself and Irene?), and emotional stress (stage fright).
One myth is that milk causes cotton mouth. This is simply not true; milk is a thick liquid, and may momentarily appear to "thicken" your saliva, but it does not cause cotton mouth.
As they say, "An ounce of prevention is worth a pound of cure." The best way to prevent cotton mouth is to drink lots of water. Seems fairly obvious, but people often severely underestimate the amount of H2O needed to avoid dehydration. The recommended amount per day is 64 ounces - and that's assuming no strenuous activity, such as exercise. An important concept of cotton mouth is that it is not how much you drink in one sitting, but how often you drink something. Taking a sip of water every 10 minutes while exercising is better than drinking a tall glass before your one-hour jog. Also, be forewarned: if you begin to feel the effects of cotton mouth, you are already dehydrated. Stop exercising immediately, drink some water, and rest for at least 15 minutes.
Another (obvious) cure is to move to a cooler area when you begin to feel cotton mouth. Exercising indoors has numerous advantages because of the temperature settings. Don't think you have to run your daily mile beneath the scorching July sun - exercising outdoors doesn't work your muscles any more than staying inside on a treadmill. Or, alternatively, you can wait until the sun has gone down (or get up before it rises!) to run at a cooler part of the day.
Finally, for all of you suffering from nervousness as you approach the podium to defend your Ph.D. thesis or accept your Nobel Prize, here's some good advice: bite your tongue! Biting the end of your tongue will give your saliva glands the impression you're about to start eating, and they'll supply you with a small but sufficient amount of saliva. Just be careful and don't lose this tip (or your own!)